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This topic will show you how to limit an MDI Child browse procedure to a single instance using messaging.

The simplest solution is to disable the menu item when the browse procedure is active. You can't do this in the menu itself--you must send a message to the Frame procedure from the browse.

First you need to declare two new events in the Global Properties, Global Data embed point:

**EVENT:DisableCustomerItem     EQUATE(401h)**

**EVENT:EnableCustomerItem      EQUATE(402h)**

(Note that user-defined events must start after 400h.)

You also need a global variable, define this in **Global Properties, Data.** Call this **GLO:MainThreadNo**, make it a BYTE.

In the Frame procedure, **WindowManager Executable Code--Init **embed, type the following:

**  GLO:MainThreadNo = THREAD()**

(Whenever an MDI procedure is STARTed, a thread number is allocated to it. We need the thread number in order to post a message to the frame procedure.)

Now, still in the frame procedure, you need to write code in the **WindowManager Executable Code--TakeWindowEvent [Priority: 2800]** embed to handle the two user-defined events that the frame procedure will receive.

**  OF EVENT:DisableCustomerItem**

**   DISABLE(?ShowCust)**

**  OF EVENT:EnableCustomerItem**

**   ENABLE(?ShowCust)**

In the Browse procedure, in the WindowManager Executable Code--Init [Priority:5600] embed, type:

**  POST(EVENT:DisableCustomerItem,,GLO:MainThreadNo)**

In the WindowManager Executable Code--Kill [Priority:8000] embed, type:

**  POST(EVENT:EnableCustomerItem,,GLO:MainThreadNo)**

Now, the first time you select the Browse Procedure from your menu, ShowCust starts up, the POST() statement executes and an **EVENT:DisableCustomerItem** is sent to the Frame procedure.

If the user then clicks on the menu again, the message is processed and the item is disabled.

As the user exits ShowCust, the **EVENT:EnableCustomerItem** message is sent to the Frame. When that message is processed the menu item is enabled again.

Why store the Frame's thread number - surely it would always be number 1? Well it might NOT be the first thread in the application.

(Thanks to Rob Mousley of Chariot Software for submitting this topic.)